1. Field of the Invention
This invention relates to an emitter coupled logic (ECL) circuit having one or more input transistors and pull-down resistors, and more particularly to an improvement of the connection of the pull-down resistors.
2. Description of the Related Art
ECL circuits are widely used in logic circuits operable at a high speed. The ECL circuits are basically composed of a differential type comparator and one or two emitter-followers. The differential type comparator has a reference transistor applied with a reference voltage at its base, at least one input transistor receiving input signals at respective bases, and a constant current source supplying a constant current to emitters of the reference and input transistors. The input signals are compared with the reference voltage.
Usually, the input signals in an ECL circuit have a high level (V.sub.H) between -0.88 and -1.025 volts and a low level (V.sub.L) between -1.62 and -1.81 volts. The reference voltage is -1.3 volts. Transistors operating in this voltage range do not reach a saturated condition and do not store any storage carrier. The emitter-follower transistor(s) is also driven in a non-saturation condition. Therefore, the ECL circuit operates at a high speed, compared to any other type of logic circuit.
ECL gates in an ECL-IC have many input transistors. However, when the ECL circuits are used as input stages in logic IC's all of the input transistors are not always used. That is, the logic IC's are designed so as to be applicable to various usage. Accordingly, the number of input transistors in the input stage ECL circuit is not selected to fit the specific usage. For example, the logic IC having an input stage of three-input ELC OR/NOR gate may be used to receive two or three input signals. If two input signals are applied to the logic IC, one input of three-input ECL OR/NOR gate must leave open. Additionally, such input open condition occurs, when a wiring to the input terminal of the logic IC is accidentally disconnected.
ECL gates in the prior art are provided with pull-down resistors to drive the non-used transistors into a cut-off condition. The pull-down resistors are respectively inserted between a base of each input transistor and a power supply terminal held at a lower potential voltage. Due to the pull-down resistor, the input transistor having a base which is not connected to any outside circuit remains in a cut-off condition. The logic operation is determined by input transistors whose bases are connected to an outside circuit. The above-explained pull-down resistors are disclosed on page 1-8 of "THE SEMICONDUCTOR DATA LIBRARY" series A, volume IV, distributed by Motorola Inc., in 1974 and page 3-4 of "ECL DATA BOOK" distributed by Fairchild Camera and Instrument Corporation in 1977.
Heretofore, logic IC was composed of relatively large-sized transistors which have a relatively deep base-emitter junction and a relatively large breakdown voltage for the base-emitter junction. The conventional ECL gates have pull-down resistors and perform their functions without any trouble. In recent years, however, the integration density of transistors in the logic IC is greatly increased. The transistors used therein necessarily become smaller in size. The base-emitter junctions are designed with a shallow depth. Due to such miniaturization, the breakdown voltage of the base-emitter junction becomes low.
The base of an input transistor having no connection to an outside circuit is reduced to the lower potential of the power supply by the pull-down resistor. The emitter thereof is held at a potential lower than the reference voltage of -1.3 volts by a base-emitter forward voltage of 0.8 volts. The lower potential of the power supply in ECL logic is about -5.0 volts. The base-emitter junction of the input transistor is backward biased with -2.9 volts. Transistors having a shallow base-emitter junction fail under these conditions. As the base-emitter junction becomes shallower, the break-down of the base-emitter junction becomes an important problem to be solved.